1. Field of the Invention
The present invention relates to an apparatus which detects the head area of a person from a picture captured by an image capturing apparatus such as a camera.
2. Description of the Related Art
Recently, attention has been paid to a function of automatically controlling the focus and exposure of a camera or the posture of the camera for panning, tilting, zooming, and the like by specifying and tracking the position of a designated person in the picture captured by an image capturing apparatus such as a camera.
The position of a person has been generally specified by detecting a face pattern of the person and tracking the movement of the pattern. Techniques of detecting a face in such an image include various schemes disclosed in M. H. Yang, D. J. Kriegman, and N. Ahuja, “Detecting Faces in Images: A Survey”, IEEE Trans. on PAMI, Vol. 24, No. 1, pp. 34-58, January, 2002. Face detection studies have widely used the AdaBoost based technique disclosed in P. Viola and M. Jones, “Robust Real-time Object Detection”, in Proc. of IEEE Workshop SCTV, July, 2001 owing to the high detection execution speed and high detection ratio. However, simply detecting and tracking a face pattern of a person are not sufficient to specify the position of the person. This is because when the person faces sideways or backwards, it may be impossible to detect a face pattern.
To detect and track a head area instead of a face, therefore, is a promising ways for compensating for the drawbacks in face pattern detection. It is possible to use, for the detection of a head area, the detection of curves by using the Hough transformation described in Duda, R. O. and P. E. Hart, “Use of the Hough Transformation to Detect Lines and Curves in Pictures”, Comm. ACM, Vol. 15, pp. 11-15 (January, 1972), the ellipse detection method described in Stan Birchfield, “Elliptical Head Tracking Using Intensity Gradients and Color Histograms”, Proc. IEEE International Conference On Computer Vision and Pattern Recognition (CVPR '98), Santa Barbara, Calif., pp. 232-237, June 1998, or the like.
In detection of a head ellipse using the Hough transformation or the technique described in Stan Birchfield, “Elliptical Head Tracking Using Intensity Gradients and Color Histograms”, Proc. IEEE International Conference On Computer Vision and Pattern Recognition (CVPR '98), Santa Barbara, Calif., pp. 232-237, June 1998, an edge having an intensity similar to that between the head and the background often exists between the face and the hair. For this reason, an edge between the face and the hair is often mistaken for a head edge. If this false detection continues, tracking of a person becomes unstable. For example, when a person faces backwards, an edge between the face and the hair disappears. As a consequence, the person is lost from tracking.